Chips, Ahoy! 197

Experiment 20: A Powerful Combination

Experiment 20: A Powerful Combination

Suppose you want to prevent other people from using your computer. I can
think of two ways to do this: using software, or using hardware. The software
would be some kind of startup program that intercepts the normal boot se-
quence and requests a password. You could certainly do it that way, but I think
it would be more fun (and more relevant to this book) to do it with hardware.
What I’m imagining is a numeric keypad requiring the user to enter a secret
combination before the computer can be switched on.

You will need:

• Numeric keypad. As specified in the shopping list at the beginning of this
  chapter, it must have a “common terminal” or “common output.” The sche-
  matic in Figure 4-82 shows what I mean. Inside the keypad, one conductor
  (which I have colored red to distinguish it from the others) connects with
  one side of every pushbutton. This conductor is “common” to all of them.
  It emerges from the keypad on an edge connector or set of pins at the
  bottom, which I’ve colored yellow.

1

4

7

2

5

8

0

3

6

9

Figure 4-82. A keypad of the type required for Experiment 20 incorporates a common
terminal connected to one side of each of the 12 pushbuttons. The wire from the common
terminal is shown red, here, to make it more easily identifiable.

• Keypads that use “matrix encoding” won’t work with the circuit that I’m
  going to describe. If the Velleman keypad, which I recommend, is unavail-
  able, and you can’t find another like it, you can use 12 separate SPST push-
  buttons. Of course, that will cost a little more.


• 74HC08 logic chip containing four AND gates. Quantity: 1.


• 74HC04 logic chip containing six inverters. Quantity: 1.


• 555 timer chip. Quantity: 1.

The Warranty Issue

If you follow this project all the way

to its conclusion, you’ll open your

desktop computer, cut a wire, and

saw a hole in the cabinet. Without a

doubt, this will void your warranty.

If this makes you nervous, here are

three options:

1. Breadboard the circuit for fun, and
   leave it at that.


2. Use the numeric keypad on some
   other device.


3. Use it on an old computer.

Figure 4-81. Caution: This just might void

your warranty.